1. Field
One or more embodiments relate to a solid-state fuel cell and a method of manufacturing the same, and more particularly, to a solid-state fuel cell including a chemical electrolyte protection layer and a method of manufacturing the same.
2. Description of the Related Art
It is desirable for a fuel cell to secure high energy conversion efficiency and price competency. In order to support anode and cathode reactions, catalysts are respectively included in the anode and the cathode of the fuel cell. In such fuel cells, if the temperature is increased, more electrochemical reaction may occur, even if the amount of catalysts remains the same. Thus, a lesser amount of catalyst is needed for a fuel cell that operates at a higher temperature, thereby improving price competency. On the other hand, in a polymer electrolyte fuel cell, an electrolyte membrane should be hydrated to obtain ionic conductivity. Accordingly, in order to avoid dehydration of the electrolyte membrane, the polymer electrolyte fuel cell should be operated at a temperature of 80° C. or less at the pressure of 1 atmospheric pressure. Thus, a relatively large amount of precious metal catalysts such as platinum may be used in a polymer electrolyte fuel cell.
In a high temperature fuel cell such as a solid oxide fuel cell (SOFC), the operating temperature is relatively high, about 800-1000° C., and thus low-priced metals such as nickel and oxides such as lanthanum strontium manganite may be used as the catalysts. However, in such high temperature fuel cells, the difference between operating temperature and starting temperature is very high and thus the initialization of high temperature fuel cells is slow. Also, stress due to the difference in thermal deformation of each material is accumulated while a high temperature fuel cell is turned off and on so that durability of the fuel cell decreases. In addition, since conductive materials (such as, for example, an INCONEL alloy), which resist oxidation/reduction reaction at high temperature, may be used as an inter-connector, the cost of materials used in manufacturing of the high temperature fuel cell increases and productivity of the fuel cell decreases. In addition, since a glass-based material may be used as a sealing material, it is difficult to manufacture the high temperature fuel cell and the high temperature fuel cell has weak resistance to shocks.
When the operating temperature of a fuel cell is below 600° C., stainless steel may be used as an inter-connector. In addition, sealing may be more easily performed at a temperature of around 200° C. However, even if the thickness of an oxygen ion conductor used in a SOFC is reduced, resistance to ionic conduction is very high at a temperature of 300° C. or less and thus, it is difficult to use the oxygen ion conductor.
On the other hand, a proton conductor such as yttrium doped barium zirconate (BYZ: Ba1-xYxZrO3-δ) has sufficient ionic conductivity at a temperature of 300° C. or less and thus may be useful as a proton conductor in a fuel cell operated at a temperature of less than 200° C. However, since carbonation may occur after BaZrO3 reacts with CO2 (BaZrO3+CO2→BaCO3+ZrO2) and since sintering of BYZ is difficult, forming a fuel cell using such a proton conductor may be difficult. However, according to the recent development of thin film processes, BYZ may be created without a sintering process through atomic layer deposition (ALD). In addition, since the ionic conductivity of BYZ is high at a low temperature, a fuel cell may be manufactured to operate at a temperature of around 80° C. by using BYZ, even when a thin film for an electrolyte having a thickness of 100 nm is manufactured. In this case, however, it is desirable to prevent carbonation resulting from the exposure of BZY to CO2. More specifically, a large amount of CO2, which is generally included in fuel, flows into a fuel cell through an anode and may carbonate BYZ used as an electrolyte. Moreover, several publications (K. D. Kreuer, Annu. Rev. Mater. Res. p. 348; Uda and Haile) disclose that BaZrO3 reacts with CO2 in the atmosphere (concentration: 330˜380 ppm) at a temperature of 300° C. or less.